Experimental method for coal desulfurization and deashing using permeation and solvating power of a supercritical fluid

ABSTRACT

An experimental method for coal desulfurization and deashing using permeation and solvating power of a supercritical fluid includes the following steps. The coal sample is ground and loaded into an extraction kettle with a cover. An inlet valve and an outlet valve of the extraction kettle are opened to circulate the supercritical CO 2  fluid in the extraction kettle. The extraction kettle is sealed. By adjusting a temperature and a pressure in the extraction kettle, the supercritical CO 2  fluid is kept at its critical point and permeates the coal sample to dissolve organic sulfur, inorganic sulfur and ash in the coal sample. The extraction kettle is depressurized, and the temperature in the extraction kettle is adjusted to gasify the supercritical CO 2  fluid. The organic sulfur, the inorganic sulfur and part of the ash are separated from the supercritical CO 2  fluid and precipitated at a bottom of the extraction kettle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority from Chinese PatentApplication No. 201911147884.2, filed on Nov. 21, 2019. The content ofthe aforementioned application, including any intervening amendmentsthereto, is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present application relates to the pre-combustion desulfurization ofcoal, and more particularly to an experimental method for coaldesulfurization and deashing using permeation and solvating power of asupercritical fluid.

BACKGROUND

China has abundant coal resources and is the largest producer andconsumer of coal in the word. However, the combustion ofsulfur-containing coals leads to high concentration of SO₂ and otherharmful gases released into the atmosphere, which has a negative impacton human health and the growth of animals and plants. In addition,emissions from coal combustion cause a series of environmental problemssuch as air pollution, acid rain and climate change. So, coaldesulfurization is urgently needed before the combustion.

Inorganic sulfur can be effectively removed from coal by physicalmethods, such as gravity separation and deashing. However, there is alarge variation of organosulfur compounds, including aliphatic sulfur,such as thioether and mercaptan, and conjugated sulfur, such asthiophene sulfur, sulfone sulfur and sulfoxide sulfur. The major coaldesulphurization techniques are physical coal cleaning, chemical andbiological methods. Each has some technical and economical limitations.For example, the physical coal cleaning is commercially deployed andwidely applied, but it can remove only the inorganic sulfur. Thechemical and biological methods can remove both the inorganic sulfur andpart of aliphatic organic sulfur, but they have limited ability toremove the organic sulfur with conjugated macromolecules, in addition,they have high processing costs and cannot be applied for industrialuse.

In some coal desulphurization methods, sulfur-fixing agents are added.However, it should be considered that the addition of these chemicalreagents influence and change the quality of the coal, and a follow-upprocess is required for removing these chemical reagents, whichcomplicates the whole process. Moreover, coal gasification andliquefaction are still under development and require more efforts tobecome applicable.

As the coal density increases, the content of the aliphatic organicsulfur like thioether and mercaptan decreases, and the content of theconjugated organic sulfur like thiophene sulfur and sulfoxide sulfurincreases, which makes the removal of the organic sulfur more difficult.

In summary, the exiting pre-combustion desulphurization techniques havethe problems as follows: 1) the organic sulfur cannot be removed inlarge amounts from the coal; 2) the process is complex and costly, andthe large-scale industrialization cannot be realized; and 3) theaddition of desulfurization agents is required during the coal treatmentfor some methods, however, the added desulfurization agents not onlyinfluence the characteristics of the coal, but also need to be removedafter the desulfurization, which leads to complicated process.

Therefore, it is urgent to develop a method for effectively removing thecomplex organosulfur compounds from the coal.

SUMMARY

Given the above, the present application provides an experimental methodfor coal desulfurization and deashing using permeation and solvatingpower of a supercritical fluid, by which complex organosulfur compoundsare effectively removed from the coal.

The technical solutions of the present application will be described asfollows.

The present application provides an experimental method for coaldesulfurization and deashing using permeation and solvating power of asupercritical fluid, comprising:

1) grinding a coal sample;

2) preparing an extraction kettle with a cover; opening an inlet valveand an outlet valve of the extraction kettle, so that the extractionkettle is connected to a container in which a supercritical CO₂ fluid isstored; circulating the supercritical CO₂ fluid in a closed system ofthe extraction kettle; introducing ethanol into a pipeline of the closedsystem by a pump to clean the pipeline; and closing the outlet valve ofthe extraction kettle to ensure an air tightness of the extractionkettle;

3) loading the coal sample obtained in step 1) into the extractionkettle; and screwing the cover of the extraction kettle to seal theextraction kettle;

4) adjusting a density of the supercritical CO₂ fluid by adjusting atemperature and a pressure in the extraction kettle, so that thesupercritical CO₂ fluid is kept at its critical point; re-adjusting thetemperature and the pressure in the extraction kettle to allow thesupercritical CO₂ fluid to permeate the coal sample and then dissolveorganic sulfur, inorganic sulfur and ash in the coal sample, therebyseparating the organic sulfur, the inorganic sulfur and the ash from thecoal sample;

5) depressurizing the extraction kettle, and adjusting the temperaturein the extraction kettle by a heat exchanger, such that thesupercritical CO₂ fluid is gasified, and the organic sulfur, theinorganic sulfur and the ash are separated from the supercritical CO₂fluid and precipitated at a bottom of the extraction kettle, therebycompleting coal desulfurization and deashing; and

6) closing the inlet valve of the extraction kettle and opening theoutlet valve of the extraction kettle to discharge gaseous CO₂ orrecycle the gaseous CO₂ by a cooling system; disconnecting a gas piping;stopping heating to allow the coal sample to cool down; opening theextraction kettle to take out the coal sample; soaking the obtained coalsample in acetone for extraction; washing the coal sample usingdeionized water and then drying the coal sample.

In the present application, a supercritical fluid technique is adoptedas a core technique, in which the adjustment of the temperature and thepressure is involved. The solubility of solute in the supercritical CO₂fluid is related to the density of the supercritical CO₂ fluid, however,the temperature and the pressure together determine the density of CO₂.By changing the temperature and the pressure, CO₂ becomes in asupercritical state and obtains the ability of selective dissolution andseparation.

The organic sulfur is effectively removed from the coal, which benefitsthe substantial reduction in dust. The experimental method of thepresent application has simple process and easy operation. It is easy torealize the control to the temperature and the pressure, which issuitable for large-scale industrialization. At the same time, CO₂ isnon-toxic and non-polluting to raw materials and has no influence to thecoal quality.

CO₂ in the supercritical state has good permeability and ability todissolve substances to be removed, in contrast, CO₂ in anon-supercritical state has low permeability and poor ability todissolve the substances to be removed. That is, the fluid in thesupercritical state can permeate into micro-pores inside the coal toselectively dissolve organic solutes in the micro-pores, and then flowsout with the organic solutes followed by rapid condensation, therebyseparating the organic solutes from the coal.

In the present application, an extraction kettle with a cover is used,such that solid materials can be frequently loaded or unloaded from thereactor under high pressure conditions. The supercritical fluid is asupercritical CO₂ fluid, which has low viscosity, good characteristicsof mass transfer, diffusibility and compressibility, and good solvatingpower for highly polar solvents. In addition, the supercritical CO₂fluid is safe, energy-saving, and pollution-free extraction solvent andthus it is used as an extracting solvent in the present application.

In some embodiments, in step 1), a particle size of the coal sample is1-3 mm or 3-6 mm.

In some embodiments, in step 4), the temperature in the extractionkettle is raised to 35-40° C., and the pressure in the extraction kettleis raised to 7-10 MPa, to make CO₂ in a supercritical state.

Compared to the prior art, the present application has the followingbeneficial effects.

1) In the experimental method of the present application, the organicsulfur and the inorganic sulfur can be removed in large amounts from thecoal, specifically, 23% organic sulfur is removed, and 70% inorganicsulfur is removed, and ash content is reduced by 32%.

2) The experimental method of the present application involves low costand simple process, and the temperature and the pressure thereof can beeasily controlled. In addition, CO₂ is readily available andcost-effective, and it can be recycled, which helps to realizelarge-scale industrialization.

3) The experimental method of the present application adopts CO₂ whichdoes not remain in products to avoid pollution to the products andeliminate a subsequent treatment, as against some existingdesulfurization methods in which desulfurization agents are adopted forcoal treatment, and the added desulfurization agents not only influencethe characteristics of the coal, but also need to be removed after thedesulfurization, which complicates the process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of an experimental method for coal desulfurizationand deashing using permeation and solvating power of a supercriticalfluid according to the present application.

FIG. 2 is a schematic diagram of a device using the experimental methodof the present application.

DETAILED DESCRIPTION OF EMBODIMENTS

The present application will be further described below with referenceto the embodiments.

An experimental method for coal desulfurization and deashing usingpermeation and solvating power of a supercritical fluid, as shown inFIGS. 1-2, includes the following steps.

1) Sample Pretreatment

A coal sample is ground until it has a particle size of 1-3 mm or 3-6mm.

2) Removal of Air and Impurities in an Extraction Kettle with a Cover

An inlet valve and an outlet valve of the extraction kettle are openedto connect the extraction kettle and a container in which asupercritical CO₂ fluid is stored. The supercritical CO₂ fluid iscirculated in a closed system of the extraction kettle. Ethanol isintroduced into a pipeline of the closed system by a pump to clean thepipeline. The outlet valve of the extraction kettle is closed to ensurean air tightness of the extraction kettle.

3) Sample Loading

The coal sample obtained in step 1) is loaded into the extractionkettle. The cover of the extraction kettle is screwed to seal theextraction kettle.

4) Desulfurization Experiment

4.1) Adjustment of Temperature and Pressure

A density of the supercritical CO₂ fluid is adjusted by adjusting atemperature and a pressure in the extraction kettle, so that thesupercritical CO₂ fluid is kept at its critical point. The temperatureand the pressure in the extraction kettle are re-adjusted such that thesupercritical CO₂ fluid permeates the coal sample and then dissolveorganic sulfur, inorganic sulfur and ash in the coal sample, therebyseparating the organic sulfur, the inorganic sulfur and the ash from thecoal sample.

4.2) Separation of the Organic Sulfur, the Inorganic Sulfur and the Ashfrom the Supercritical CO₂ Fluid

The extraction kettle is depressurized, and the temperature in theextraction kettle is adjusted by a heat exchanger, such that thesupercritical CO₂ fluid is gasified, and the organic sulfur, theinorganic sulfur and part of the ash are separated from thesupercritical CO₂ fluid and precipitated at a bottom of the extractionkettle, thereby completing coal desulfurization and deashing.

5) Sample Collection and Subsequent Processing

After the experiment, the inlet valve of the extraction kettle is closedand the outlet valve of the extraction kettle is opened to dischargegaseous CO₂ or recycle the gaseous CO₂ by a cooling system;disconnecting a gas piping. A gas piping is disconnected. Heating stopsand the coal sample gradually cools down. The extraction kettle isopened to take out the coal sample. The obtained coal sample is soakedin acetone for extraction and washed using deionized water followed bydrying the coal sample for testing the coal sample.

A raw coal from Gaoyang, Baoding City, Hebei province, China was used inan embodiment as follows. Organic sulfur was removed from the raw coalusing a SC—CO₂ method.

1) Sample Pretreatment

A coal sample was ground until it had a particle size of 1-3 mm or 3-6mm.

2) Removal of Air and Impurities in an Extraction Kettle with a Cover

An inlet valve and an outlet valve of the extraction kettle were openedto connect the extraction kettle and a CO₂ cylinder, where a pressure ofthe CO₂ cylinder at an outlet thereof was kept within 5-6 MPa. Thesupercritical CO₂ fluid was circulated in a closed system of theextraction kettle, and at the same time, the closed system was kept at arequired pressure. Ethanol was introduced into a pipeline of the closedsystem by a pump to clean the pipeline. The outlet valve of theextraction kettle was closed to ensure an air tightness of theextraction kettle.

3) Sample Loading

The coal sample obtained in step 1) was weighted and loaded into asample container, and then a filter, a gasket, a porous plate and athreaded ring were placed into the sample container in sequence. Aspecial handle was used to place the sample container into a samplecavity of the extraction kettle. Specifically, the coal sample in thesample container was about 2-3 cm away from the filter arranged abovethe coal sample to ensure that the sample container was not completelyfilled with the coal sample. Preferably, the gasket was O-shaped, andthe porous plate was made of steel. The threaded ring was screwed intothe sample container for the compressing purpose. A volume of the samplecavity was 1 L.

Subsequently, the cover of the extraction kettle was screwed to seal theextraction kettle.

4) Desulfurization Experiment

A device using the experimental method of the present application wasprovided with a plurality of circulating water tanks. Before theoperation of the device, the circulating water tanks were filled withwater, and then the water was heated by an electric heating tube. Theheated water was transferred by a circulating pump for heating theextraction kettle and keeping it at a required temperature. The devicewas also provided with a cool box, and a 30% by weight of aqueous glycolsolution was stored in the cool box. The cooling water source was keptunblocked relative to a water-cooled unit.

4.1) Adjustment of Temperature and Pressure

In the SC—CO₂ process, a density of the supercritical CO₂ fluid wasadjusted by adjusting a temperature and a pressure in the extractionkettle, so that the supercritical CO₂ fluid was kept at its criticalpoint. The temperature and the pressure in the extraction kettle werere-adjusted such that the supercritical CO₂ fluid dissolved andseparated substances to be removed. During which, the pressure in theextraction kettle and the extraction kettle were always kept at arequired value, until the coal sample was taken out of the device.

4.2) Separation of a Solute from the Supercritical CO₂ Fluid

The supercritical CO₂ fluid permeated the coal sample within a set timeto dissolve organic sulfur and inorganic sulfur in the coal sample. Theremoval of the inorganic sulfur was accompanied by a decrease in an ashcontent. After decompression, the supercritical CO₂ fluid was separatedfrom the organic sulfur, thereby completing the coal desulfurization anddeashing.

4.3) Depressurization and Heat Exchange

The extraction kettle was depressurized and a temperature in theextraction kettle was adjusted via a heat exchanger, so as to gasify thesupercritical CO₂ fluid and precipitate the separated liquid organicsulfur at a bottom of the extraction kettle. Gaseous CO₂ was dischargedor recycled by a cooling system.

5) Sample Collection and Subsequent Processing

After the experiment, a refrigerator pump stopped working, and a CO₂plunger pump also stopped working. Preferably, the CO₂ plunger pump wasa high pressure pump. In addition, the heating and circulation ofcirculating water were stopped. The inlet valve of the extraction kettlewas closed and the outlet valve of the extraction kettle was opened torelease gaseous CO₂ in the extraction kettle. A gas piping wasdisconnected. Heating stopped and the coal sample gradually cooled down.The extraction kettle was opened to take out the coal sample. Theobtained coal sample was soaked in acetone for extraction and washedusing deionized water followed by drying the coal sample for nexttesting.

According to the comparison between test data after the reaction andtest data before the reaction, the content of the sulfur and the ash inthe coal is significantly reduced, and the cohesiveness and calorificvalue are basically unchanged, which indicates that the organic sulfuris effectively separated, and at the same time, the method of thepresent application does not affect the quality of the coal.

Comparative experiments are made by changing various variables, such asthe temperature, the pressure, the treatment time. The results are shownin Table 1 as follows.

TABLE 1 Sulfur Ash Volatile Coke Slag Heat Temperature Pressure TimeContent Content Content Characteristic Cohesiveness value Raw 2.26 10.5718.62 5 68.03 31.54 sample Sample 1 35 10 2 2.04 7.22 19.38 4 60.8930.74 Sample 2 35 10 5 1.80 7.14 18.83 5 58.01 32.05 Sample 3 35 10 141.69 6.56 19.19 5 65.93 31.74 Sample 4 35 10 24 1.85 6.48 16.87 5 63.0931.70 Sample 5 37 8 8 1.78 6.84 19.02 5 69.17 31.93 Sample 6 37 9 141.75 6.83 19.03 5 63.48 32.00 Sample 7 37 8 24 1.80 6.30 18.92 5 66.7532.31

The above-mentioned is only some preferred embodiments. It should benoted that any improvements and modifications may be made by thoseskilled in the art without departing from the principle of thedisclosure, and shall fall within the scope of the present disclosure.

What is claimed is:
 1. An experimental method for coal desulfurizationand deashing using permeation and solvating power of a supercriticalfluid, comprising: 1) grinding a coal sample; 2) preparing an extractionkettle with a cover; opening an inlet valve and an outlet valve of theextraction kettle, so that the extraction kettle is connected to acontainer in which a supercritical CO₂ fluid is stored; circulating thesupercritical CO₂ fluid in a closed system of the extraction kettle;introducing ethanol into a pipeline of the closed system by a pump toclean the pipeline; and closing the outlet valve of the extractionkettle to ensure an air tightness of the extraction kettle; 3) loadingthe coal sample obtained in step 1) into the extraction kettle; andscrewing the cover of the extraction kettle to seal the extractionkettle; 4) adjusting a density of the supercritical CO₂ fluid byadjusting a temperature and a pressure in the extraction kettle, so thatthe supercritical CO₂ fluid is kept at its critical point; re-adjustingthe temperature and the pressure in the extraction kettle to allow thesupercritical CO₂ fluid to permeate the coal sample and then dissolveorganic sulfur, inorganic sulfur and ash in the coal sample, therebyseparating the organic sulfur, the inorganic sulfur and the ash from thecoal sample; 5) depressurizing the extraction kettle, and adjusting thetemperature in the extraction kettle by a heat exchanger, such that thesupercritical CO₂ fluid is gasified, and the organic sulfur, theinorganic sulfur and part of the ash are separated from thesupercritical CO₂ fluid and precipitated at a bottom of the extractionkettle, thereby completing coal desulfurization and deashing; and 6)closing the inlet valve of the extraction kettle and opening the outletvalve of the extraction kettle to discharge gaseous CO₂ or recycle thegaseous CO₂ by a cooling system; disconnecting a gas piping; stoppingheating to allow the coal sample to cool down; opening the extractionkettle to take out the coal sample; soaking the obtained coal sample inacetone for extraction; and washing the coal sample using deionizedwater and then drying the coal sample.
 2. The experimental method ofclaim 1, wherein in step 1), a particle size of the coal sample is 1-3mm or 3-6 mm.
 3. The experimental method of claim 1, wherein in step 4),the temperature in the extraction kettle is raised to 35-40° C., and thepressure in the extraction kettle is raised to 7-10 MPa, to make CO₂ ina supercritical state.